How do you test that a Python function throws an exception

Question

How does one write a unittest that fails only if a function doesn t throw an expected exception

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As I haven t seen any detailed explanation on how to check if we got a specific exception among a list of accepted one using context manager  or other exception details I will add mine  checked on python 3 8   If I just want to check that function is raising for instance TypeError  I would write  with self assertRaises TypeError       function raising some exception parameters   If I want to check that function is raising either TypeError or IndexError  I would write  with self assertRaises  TypeError IndexError        function raising some exception parameters   And if I want even more details about the Exception raised I could catch it in a context like this    Here I catch any exception     with self assertRaises Exception  as e      function raising some exception parameters     Here I check actual exception type  but I could   check anything else about that specific exception    like it s actual message or values stored in the exception  self assertTrue type e exception  in  TypeError MatrixIsSingular

User · Answer

You can use assertRaises from the unittest module  import unittest  class TestClass      def raises exception self       raise Exception  test    class MyTestCase unittest TestCase     def test if method raises correct exception self       test class   TestClass         note that you dont use    when passing the method to assertRaises     self assertRaises Exception  test class raises exception

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The code in my previous answer can be simplified to   def test afunction throws exception self       self assertRaises ExpectedException  afunction    And if afunction takes arguments  just pass them into assertRaises like this   def test afunction throws exception self       self assertRaises ExpectedException  afunction  arg1  arg2

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I just discovered that the Mock library provides an assertRaisesWithMessage   method  in its unittest TestCase subclass   which will check not only that the expected exception is raised  but also that it is raised with the expected message   from testcase import TestCase  import mymod  class MyTestCase TestCase       def test1 self           self assertRaisesWithMessage SomeCoolException                                        expected message                                        mymod myfunc

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You can use context manager to run the faulty function and assert it raises the exception with a certain message using assertRaisesMessage with self assertRaisesMessage SomeException  Some error message e g 404 Not Found        faulty funtion

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While all the answers are perfectly fine  I was looking for a way to test if a function raised an exception without relying on unit testing frameworks and having to write test classes   I ended up writing the following   def assert error e  x       try          e x      except          return     raise AssertionError    def failing function x       raise ValueError    def dummy function x       return x  if   name       main         assert error failing function  0      assert error dummy function  0    And it fails on the right line    Traceback  most recent call last     File  assert error py   line 16  in  lt module gt      assert error dummy function  0    File  assert error py   line 6  in assert error     raise AssertionError   AssertionError

User · Answer

How do you test that a Python function throws an exception       How does one write a test that fails only if a function doesn t throw   an expected exception    Short Answer   Use the self assertRaises method as a context manager       def test 1 cannot add int and str self           with self assertRaises TypeError               1    1    Demonstration  The best practice approach is fairly easy to demonstrate in a Python shell    The unittest library  In Python 2 7 or 3   import unittest   In Python 2 6  you can install a backport of 2 7 s unittest library  called unittest2  and just alias that as unittest   import unittest2 as unittest   Example tests  Now  paste into your Python shell the following test of Python s type-safety   class MyTestCase unittest TestCase       def test 1 cannot add int and str self           with self assertRaises TypeError               1    1      def test 2 cannot add int and str self           import operator         self assertRaises TypeError  operator add  1   1     Test one uses assertRaises as a context manager  which ensures that the error is properly caught and cleaned up  while recorded    We could also write it without the context manager  see test two  The first argument would be the error type you expect to raise  the second argument  the function you are testing  and the remaining args and keyword args will be passed to that function    I think it s far more simple  readable  and maintainable to just to use the context manager   Running the tests  To run the tests   unittest main exit False    In Python 2 6  you ll probably need the following   unittest TextTestRunner   run unittest TestLoader   loadTestsFromTestCase MyTestCase     And your terminal should output the following      ---------------------------------------------------------------------- Ran 2 tests in 0 007s  OK  lt unittest2 runner TextTestResult run 2 errors 0 failures 0 gt    And we see that as we expect  attempting to add a 1 and a  1  result in a TypeError     For more verbose output  try this   unittest TextTestRunner verbosity 2  run unittest TestLoader   loadTestsFromTestCase MyTestCase

User · Answer

I just discovered that the Mock library provides an assertRaisesWithMessage   method  in its unittest TestCase subclass   which will check not only that the expected exception is raised  but also that it is raised with the expected message   from testcase import TestCase  import mymod  class MyTestCase TestCase       def test1 self           self assertRaisesWithMessage SomeCoolException                                        expected message                                        mymod myfunc

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Your code should follow this pattern  this is a unittest module style test    def test afunction throws exception self       try          afunction       except ExpectedException          pass     except Exception         self fail  unexpected exception raised       else         self fail  ExpectedException not raised     On Python  lt  2 7 this construct is useful for checking for specific values in the expected exception  The unittest function assertRaises only checks if an exception was raised

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For await async aiounittest there is a slightly different pattern  https   aiounittest readthedocs io en latest asynctestcase html aiounittest AsyncTestCase async def test await async fail self       with self assertRaises Exception  as e          await async one

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Since Python 2 7 you can use context manager to get ahold of the actual Exception object thrown   import unittest  def broken function        raise Exception  This is broken    class MyTestCase unittest TestCase       def test self           with self assertRaises Exception  as context              broken function            self assertTrue  This is broken  in context exception   if   name         main         unittest main     http   docs python org dev library unittest html unittest TestCase assertRaises    In Python 3 5  you have to wrap context exception in str  otherwise you ll get a TypeError  self assertTrue  This is broken  in str context exception

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You can build your own contextmanager to check if the exception was raised   import contextlib   contextlib contextmanager def raises exception       try          yield      except exception as e          assert True     else          assert False   And then you can use raises like this   with raises Exception       print  Hola     Calls assert False  with raises Exception       raise Exception    Calls assert True   If you are using pytest  this thing is implemented already  You can do pytest raises Exception    Example   def test div zero        with pytest raises ZeroDivisionError           1 0   And the result   pigueiras pigueiras  py test                   test session starts                   platform linux2 -- Python 2 6 6 -- py-1 4 20 -- pytest-2 5 2 --  usr bin python collected 1 items   tests test div zero py 6  test div zero PASSED

User · Answer

Use TestCase assertRaises  or TestCase failUnlessRaises  from the unittest module  for example   import mymod  class MyTestCase unittest TestCase       def test1 self           self assertRaises SomeCoolException  mymod myfunc

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from  http   www lengrand fr 2011 12 pythonunittest-assertraises-raises-error   First  here is the corresponding  still dum  p  function in file dum function py    def square value a             Returns the square value of a            try         out   a a    except TypeError         raise TypeError  Input should be a string        return out   Here is the test to be performed  only this test is inserted    import dum function as df   import function module import unittest class Test unittest TestCase                The class inherits from unittest              def setUp self                     This method is called before each test                   self false int    A      def tearDown self                     This method is called after each test                   pass        ---             TESTS    def test square value self            assertRaises excClass  callableObj  prototype        self assertRaises TypeError  df square value self false int       if   name         main            unittest main     We are now ready to test our function  Here is what happens when trying to run the test                                                                           ERROR  test square value    main   Test  ---------------------------------------------------------------------- Traceback  most recent call last     File  test dum function py   line 22  in test square value     self assertRaises TypeError  df square value self false int     File   home jlengrand Desktop function py   line 8  in square value     raise TypeError  Input should be a string    TypeError  Input should be a string   ---------------------------------------------------------------------- Ran 1 test in 0 000s  FAILED  errors 1    The TypeError is actullay raised  and generates a test failure  The problem is that this is exactly the behavior we wanted  s   To avoid this error  simply run the function using lambda in the test call    self assertRaises TypeError  lambda  df square value self false int     The final output    ---------------------------------------------------------------------- Ran 1 test in 0 000s  OK   Perfect        and for me is perfect too    Thansk a lot Mr  Julien Lengrand-Lambert    This test assert actually returns a false positive  That happens because the  lambda inside the  assertRaises  is the unit that raises type error and not the tested function

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There are a lot of answers here  The code shows how we can create an Exception  how we can use that exception in our methods  and finally  how you can verify in a unit test  the correct exceptions being raised   import unittest  class DeviceException Exception       def   init   self  msg  code           self msg   msg         self code   code     def   str   self           return repr  Error         format self code  self msg    class MyDevice object       def   init   self           self name    DefaultName       def setParameter self  param  value           if isinstance value  str               setattr self  param   value          else              raise DeviceException  Incorrect type of argument passed  Name expects a string   100001       def getParameter self  param           return getattr self  param   class TestMyDevice unittest TestCase        def setUp self           self dev1   MyDevice        def tearDown self           del self dev1      def test name self               Test for valid input for name parameter              self dev1 setParameter  name    MyDevice           name   self dev1 getParameter  name           self assertEqual name   MyDevice        def test invalid name self               Test to check if error is raised if invalid type of input is provided              self assertRaises DeviceException  self dev1 setParameter   name   1234       def test exception message self               Test to check if correct exception message and code is raised when incorrect value is passed              with self assertRaises DeviceException  as cm              self dev1 setParameter  name   1234          self assertEqual cm exception msg   Incorrect type of argument passed  Name expects a string    mismatch in expected error message           self assertEqual cm exception code  100001   mismatch in expected error code     if   name         main         unittest main

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I use doctest 1  almost everywhere because I like the fact that I document and test my functions at the same time   Have a look at this code   def throw up something  gowrong False                gt  gt  gt  throw up  Fish n Chips       Traceback  most recent call last               Exception  Fish n Chips       gt  gt  gt  throw up  Fish n Chips   gowrong True       I feel fine               if gowrong          return  I feel fine       raise Exception something   if   name         main         import doctest     doctest testmod     If you put this example in a module and run it from the command line both test cases are evaluated and checked    1  Python documentation  23 2 doctest -- Test interactive Python examples

[python] How do you test that a Python function throws an exception?

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